The known detection tools are essentially constituted by a detector, a collimator and information technology means for processing.
The collimator makes it possible to select the photons arriving on the detector. It is formed by channels delimited by septa.
The detector may comprise a scintillator material, such as Cesium Iodide, Sodium Iodide, Lanthanum Bromide (LaBr3) or Bismuth Germanate (BGO), which is associated with photodetectors, for example an array of photodiodes. Alternatively, the detector comprises at least one semiconductor detector material, for example CdTe or CdZnTe, capable of being polarized by a cathode and an anode, these electrodes generally being disposed on two opposite faces of a block of semiconductor material. This is then referred to as a semiconductor detector.
When a photon penetrates the semiconductor material and interacts with it, all or some of its energy is transferred to charge carriers (electron-hole pairs) in the semiconductor material. Because the detector is polarized, the charge carriers migrate towards the electrodes (including the anode). They are collected there to produce an electrical signal. This electrical signal, which is a succession of pulses of which the amplitude is proportional to the energy deposited by a photon when an interaction occurs, is then processed. According to the nature of the detector, the signal is collected solely at the anode (this is the most frequent case), solely at the cathode, or at both electrodes. A semiconductor detector usually comprises a plurality of physical pixels, each physical pixel corresponding to a circuit for charge collection by an electrode.
Three operating modes are known for detection tools with a semiconductor detector: the integration mode, the spectrometric mode and the photon counting mode.
In the integration mode, integration type electronics measures the current coming from each electrode for a given period of time, typically a few hundreds of μs. This current is the sum of the dark current, part of the current created by the incident radiation during that period and part of the current created during the preceding period, the latter being called smearing of the signal). The integration mode is well-adapted for radiology although hindered by the dark current and the smearing of the signals. In an X-ray scanner, with rapid variations in incident flux of a few decades, the smearing of the signals is prohibitive and the integration mode cannot function.
In the spectrometric mode, the current output from each electrode is amplified by a charge preamplifier and shaped with a time constant of the order of 1 μs. The measurement of this charge represents the energy of the incident photon. The spectrometric mode enables precise measurement of the energy of the incident photons but is not sufficiently fast for an X-ray scanner type application in which the incident stream of photons is greater than 109 photons/s·mm2.
In the photon counting mode, the current output from each electrode is amplified by a current preamplifier and is compared with a threshold, referred to as counting threshold. This counting threshold makes it possible to discriminate a low amplitude interaction, which will be rejected, from a significant amplitude interaction, which will be taken into account. Typically, the counting threshold may be equivalent to an energy of 25 keV, only the interactions releasing greater energy being taken into account, and thus counted.
The typical duration of the pulses of a signal for an X-ray scanner type application is of the order of 5 to 15 ns. If the pulse amplitude studied is greater than the counting threshold, a counter increments. The counting mode for the photons is compatible with high fluxes, the detector count rate being in particular greater than 1 Mcount/s/mm2. At such count rates, there is no question of making an accurate measurement of the energy deposited by each interaction. Common devices merely make a measurement of the amplitude, that is to say the maximum level, of each pulse produced by the detector.
However, at such count rates, perturbations may affect the detector, affecting the stability of the detector response. Thus, for the same energy released in the detector, the shape of the pulses may drift, the pulses being less high and longer. It can be understood that mere amplitude thresholding is reaching its limits. As a matter of fact, interactions releasing the same energy may give rise to pulses of which the maximum amplitude is different, which leads to a degradation in the energy resolution.
It has for example been found that when a detector is exposed to an intense and constant incident flux, the number of pulses counted, that is to say the number of pulses exceeding a predetermined amplitude threshold, reduces.
Such a drift may prove to be critical, in particular in the case of an X-ray scanner in which small variations of an electrode relative to another of a few per thousand lead to artefacts when reconstructing an image.